NO. 1454. TWO XE W XA I A DS—BA RSTCH. 395 



walled, and beset with several rows of long, thick, conical papilla?. 

 Dorsal syphon 6 mm. long, moderately thick walled, and not papillose. 

 Outer gills much smaller than inner, terminating somewhat posterior 

 to the basal attachment of the inner gills; roughly they represent an 

 isosceles triangle, the base of which constitutes the line of attachment; 

 the anterior edge is a little less regular, not quite as straight as the 

 posterior side, and falls off just a trifle more abruptW. The inner 

 gills correspond at the posterior end with the outer gills in size, shape, 

 and slope, but they extend forward in a slightly curved line, almost 

 undiminished in width to the labial palpi, where they are suddenl}' 

 deflected dorsally, their anterior margin being very short. Labial 

 palpi small, with the free edge somewhat sinuous, agreeing with each 

 other in shape and size, but the outer one is attached to the mantle in 

 such a way that it probably presents only half the free surface on its 

 outside that the inner presents on its inside. The space between them 

 is equal. Foot and body of the animal small, the entire length of the 

 bodj'^ at the base of the gills being only 129 mm. , while the entire length 

 of the animal from the anterior mantle edge to the posterior mantle 

 edge is 52.5 mm. The posterior half of the mantle is ash colored, 

 deepest at the posterior edge, fading gradually anteriorly. 



The shell in a general waj^ recalls Dlplodon easpehlanav Philippi and 

 Diplodon frenzellii Ihering, but can readily be distinguished from 

 them by its narrower outline. 



This species is reported to be very abundant in a small lake on Vic- 

 toria Island, in Lake Nahuel Huape, Argentina, where the natives are 

 said to roast and eat them. 



One of the specimens has four lateral teeth, two in the right valve, 

 instead of one; the ventral one of these, the extra one, is almost as 

 strong as the dorsal member. 



